The present invention relates to methods and apparatus for increasing the bulk density of aerated powders. By way of example only, the invention can be utilized to increase the bulk density of highly aerated, free-flowing inorganic metal oxide powders with considerable commercial significance, for example, titanium dioxide pigments, complex metal oxides of the type presently being employed in primary and secondary rechargeable batteries (typically comprising lithium metal oxides) and blends of such complex metal oxides with various other components of a cathode composition of a battery.
Handling and containing fine, highly aerated powders can be problematic in many respects. For example, filling a bag or other container to capacity with a highly treated titanium dioxide pigment (for example, one designed for use in water-based latex paints) can be difficult to accomplish in an efficient manner without first deaerating the pigment. Due to the relatively low bulk density of the pigment, the container can generally be filled to only 80 to 90% of its capacity. On standing, air entrapped in the pigment will slowly rise through the tortuous pathways defined between gravitationally settling pigment particles, in the process increasing the bulk density of the pigment and allowing additional pigment to be added to the container. However, in a continuous manufacturing and packaging process, the additional time and handling required to fill the container to capacity makes the process inefficient. Further, it can be difficult to impart a consistent, predetermined amount of pigment to each bag in a continuous bagging process. Similarly, filling a battery compartment or shell to capacity or with an exact amount of battery-active material (e.g., cathode material) can be difficult to achieve due to air entrapped in the material.
Various processes have been utilized to deaerate and compact a free-flowing powder. For example, the powder container has been placed on top of a device that allows the container to be shaken and/or vibrated as the container is filled. A similar technique involves placing a vibrating rod into the container in order to cause entrapped air to dissipate. Additional methods utilized in the past include a compression device for compressing the container and powder therein in order to squeeze out air entrained in the powder, and placing a porous pipe connected to a vacuum system into the container during the filling process to evacuate the entrained air. All of these processes have serious drawbacks. For example, although removing entrained air with a porous pipe works for a short time, the pores in the pipe ultimately become blocked due to the fine particle size of many powders.
One technique that has been used commercially over the years is vacuum densification. In a vacuum densification process, the powder to be deaerated is placed in a container that is connected to a vacuum source. A vacuum is then pulled to whatever level is desired. Upon attaining the desired vacuum level, the valve controlling the vacuum source is closed and a second valve into the container is opened allowing the pressure within the container to rapidly equilibrate back to atmospheric pressure. This process causes the powder to compact.
Unfortunately, like the other powder deaeration processes utilized heretofore, vacuum densification has its drawbacks. For example, vacuum systems require an elaborate filter system and are generally somewhat expensive to put in place. Many powder manufacturing plants do not otherwise have vacuum systems in place. Also, vacuum systems are limited to atmospheric pressure (approximately 15 psig (1 kg/sq. cm, gauge)).